This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c. to d.c. voltage conversion, featuring provisions for attainment of a higher power factor, and to a method of driving the switching power supply.
The d.c.-to-d.c. converter in general employs as its source of d.c. voltage an a.c.-to-d.c. converter circuit for connection mostly to a commercial a.c. power supply. A conversion from an alternating to a direct current is possible by a combination of a rectifying circuit having a diode connected to an a.c. power supply, and a smoothing capacitor connected to the rectifying circuit. This type of rectifying and smoothing circuit possesses the disadvantage of a low power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the incoming a.c. voltage of sinusoidal waveform. Another drawback is that it is in capable of adjustably varying the d.c. output voltage.
The concept of reconstructing the a.c.-to-d.c. converter circuit for a higher power factor is itself not new in the art. It has been suggested, for example, to connect an electronic switch between the pair of outputs of the rectifier circuit via an inductor, and to connect the smoothing capacitor in parallel with the switch via a diode. The switch is driven at a repetition frequency (e.g. 20 kHz) that is higher than the frequency (e.g. 50 Hz) of the input a.c. voltage. The inductor is connected between the outputs of the rectifier circuit during the conducting periods of the switch, with consequent current flow therethrough. The peak value of the current through the inductor varies in proportion with the instantaneous value of the sinusoidal a.c. voltage. A higher power factor is attained as the current at the input stage of the rectifier circuit more closely approximates a sinusoidal wave.
As an additional outcome of the power factor improvement of the a.c.-to-d.c. converter circuit, the smoothing capacitor is charged by the resultant of the output voltage of the rectifier circuit and the inductor voltage during the nonconducting periods of the switch. The smoothing capacitor is thus charged to a voltage higher than the output voltage of the rectifier circuit.
There has, however, been a problem left unsolved with the switching power supply comprising an a.c.-to-d.c. converter circuit with an improved power factor, and a d.c.-to-d.c. converter circuit utilizing the a.c.-to-d.c. converter circuit as power supply. The problem arose from the fact that the switch in the a.c.-to-d.c. converter circuit and that in the d.c.-to-d.c. converter circuit started to be driven practically simultaneously when the smoothing capacitor in the a.c.-to-d.c. converter was charged to a certain level. This may be restated that the switch in the d.c.-to-d.c. converter circuit started to be driven before commencement of the step-up charging of the smoothing capacitor due to the on-off operation of the switch in the a.c.-to-d.c. converter circuit. The possible result was an insufficient supply of input voltage to the d.c.-to-d.c. converter circuit. The output voltage of the d.c.-to-d.c. converter circuit sometimes failed to rise to the required level, causing trouble in the operation of the load to the required level, causing trouble in the operation of the load connected thereto in the worst case.
It might be contemplated to make the smoothing capacitor in the a.c.-to-d.c. converter circuit large enough in capacity to alleviate the inconvenience pointed out above. This solution is objectionable because of a costly and bulky smoothing capacitor required, which adds substantively to the cost and size of the power supply itself.